1. Field of the Invention
The present invention relates to a field effect transistor that uses a III-nitride semiconductor, and more particularly to a field effect transistor that realizes normally-off operation, and has high breakdown voltage and large current operation.
2. Description of the Related Art
Due to the intrinsic properties of the material, III-nitride semiconductors such as GaN have higher breakdown voltage and larger current density than that of silicon semiconductors, and are capable of high-temperature operation, which makes them being expected as power devices.
GaN based MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) (refer to non-patent document 1) and AlGaN/GaN based HFETs (Hetero junction Field Effect Transistor) have been generally proposed as field effect transistors that use III-nitrides.
These GaN based field effect transistors have a higher breakdown electric field and higher saturation velocity than that of Si semiconductors or conventional III-V compound semiconductors such as GaAs and InP, so are especially suitable as power devices.
In addition, GaN based MOSFETs are easily capable of normally-Off operation, so when a problem occurs in a power-supply circuit, operation proceeds in a safe direction, thus they are suitable as power devices from the aspect of fail-safe operation as well. In the GaN based MOSFETs that are described in non-patent document 1, a 940 V breakdown voltage was achieved by forming a RESURF (REduced SURface Field) layer on the semiconductor layer that reduces the surface field.
Moreover, the AlGaN/GaN based HFETs that are described in non-patent document 2 are structured with semiconductor layers being stacked on a carrier drifting layer consisted of undoped AlGaN in the order of a barrier layer consisted of an undoped or n-type AlGaN having a lattice constant that is smaller than that of the carrier drifting layer, a threshold control layer consisted of AlGaN having a lattice constant that is the same as that of the carrier drifting layer, and a carrier induced layer consisted of undoped or n-type AlGaN. Generally, normally-off operation is difficult for an AlGaN based HFET, however, by forming a gate electrode in a recess that is formed by removing a part of the carrier induction layer and a part of the barrier layer so that the film thickness of the barrier layer is equal to or less than the critical thickness, normally-off operation is made possible, and by controlling the film thickness of the barrier layer at the atomic layer level, it is possible to keep fluctuation in the threshold voltage small. Also, ON resistance is reduced by providing a carrier induction layer.
In the case of a Si power device, a method is known in which a body electrode is placed in a p+-type layer in order to realize high breakdown voltage. The breakdown voltage can be improved by removing from this body electrode the holes from the electron-hole pairs that are generated by the avalanche phenomenon that occurs when the drain voltage is increased. However, in the case of GaN, selectively forming a high-concentration p+-type layer by a method such as ion implantation method is extremely difficult. In a report of a p-type layer formed by ion implantation, the sheet carrier density of the holes is approximately 7×1012 cm−2 (refer to non-patent document 3).    Reference Documents: Non-Patent Documents    Non-patent document 1: W. Huang, T. Khan, T. P. Chow, “Enhancement-Mode n-Channel GaN MOSFETs on p and n-GaN/Sapphire Substrates,” 18th International Symposium on Power Semiconductor Devices and ICs (ISPSD) 2006 (Italy), 10-1.    Non-patent document 2: M. Kuraguchi et al., “Normally-off GaN-MISFET with well-controlled threshold voltage,” International Workshop on Nitride Semiconductors 2006 (IWN2006), Oct. 22-27, 2006, Kyoto, Japan, WeED1-4.    Non-patent document 3: Wilson R. G. et al., “Redistribution and activation of implanted S, Se, Te, Be, Mg and C in GaN,” Journal of Vacuum Science and Technology, A17, 1226 (1999).
However, in a power device it is necessary to enable normally-off operation by making the breakdown voltage higher and by making the threshold voltage of the gate electrode higher. For example, in order to be applicable in a power-supply circuit of an automobile, a drain breakdown voltage of approximately 1200 V, and a threshold voltage of +3 V or greater are required. The GaN based MOSFET of non-patent document 1 has a breakdown voltage of 940 V; however, for use in a power-supply circuit of an automobile, there is a problem in that even higher breakdown voltage is necessary.
Moreover, the AlGaN/GaN based HFET of non-patent document 2 enables normally-off operation, and is capable of controlling the threshold voltage such that there is little fluctuation; however, the threshold voltage is only about +1 V, which is less than the required +3 V threshold voltage for a power device.
Furthermore, in order to realize a GaN based power device by using a method in which an electrode is disposed in a p+-type layer and holes are removed as in an Si power device, it is necessary to form a p+-type layer having a sheet carrier density of 1×1015 cm−2 at least, however, there is a problem in that a sheet carrier density of 7×1012 cm−2 of a p+-type layer that can be formed in GaN as given in non-patent document 3 is too low. Also, it is even more difficult to selectively form a high-concentration p+-type layer.
In consideration of the problems described above, the object of the present invention is to provide a III-nitride semiconductor field effect transistor that is capable of normally-off operation, high breakdown voltage and large current.